In a telecommunications network employing the IEEE 1588 standard, a common time is distributed throughout the network so that network elements within the network are synchronized. According to IEEE 1588-2002, the first implementation of IEEE 1588, a grandmaster clock on a network element transmits timing information to a boundary clock on another network element using PTP (Precision Time Protocol) messages, typically conveying such information to the boundary clock on the order of several times a second. When the boundary clock receives timing information the boundary clock corrects its own local clock with an offset derived from the time stamps in the various PTP messages used to convey the timing information. The boundary clock then transmits its own timing information to other network elements, including possibly other boundary clocks. A clock that only receives timing information and does not convey it onward using IEEE 1588 is a slave clock. The clock delivering timing information to a second clock is referred to as the parent clock of the second clock. On any of these clocks, a port that receives timing information from another clock is referred to as a slave port, and a port that transmits timing information to another clock is referred to as a master port.
A boundary or slave clock may receive supervisory information from more than one clock. In such a case the boundary or slave clock selects one of these clocks as its parent clock by using a “best master clock” algorithm. The boundary clock or slave clock then uses the timing information received from the parent clock to adjust its local clock.
In IEEE 1588-2008, version 2 of IEEE 1588, transparent clocks are introduced. A transparent clock processes PTP messages that transit the transparent clock as they flow from a master port toward a slave port. The transparent clock does not terminate the PTP messages but augments each. PTP message with the time the PTP message requires to traverse the network element containing the transparent clock.
As part of the PTP communications grandmaster and boundary clocks distribute supervisory information using Announce messages. These Announce messages indicate the quality of the grandmaster clock which is the source of timing into the network as well as the distance of the clock from the grandmaster. For example, a boundary clock whose parent clock is the grandmaster would send Announce messages indicating the quality of the grandmaster clock and a distance of one. The grandmaster clock quality information includes an estimate by the grandmaster clock of its own accuracy and stability. Both of these are indicators of how good the clock is, and may be static or dynamic measures, A transparent clock passes Announce messages unmodified. If a boundary clock or transparent clock receives Announce messages from more than one clock, the clock can use the information in the Announce messages in its implementation of the Best Master Clock Algorithm in order to select which of these clocks to use as its parent clock.
In both cases, the boundary clock or the transparent clock (referred to herein collectively as an “intermediate clock”) may initially introduce errors into the timing information. A boundary clock that has recently booted up takes a measurable amount of time to synchronize its own clock with that of its parent clock. The PTP messages transmitted by the boundary clock may therefore not accurately reflect the time signals emanating from the grandmaster clock. Similarly, for a transparent clock its measurement of the transit time of packets through the network element containing the transparent clock may not have stabilized if the network element has recently booted up and the local clock of the transparent clock has not yet synchronized with the grandmaster clock.
In either case, the timing information gleaned by an end application may not match the timing information generated by the grandmaster clock. The Announce messages reaching an end application indicate the accuracy and stability of the grandmaster clock. The Announce messages are silent however regarding the potential decrease in accuracy arising from the presence of boundary clocks and transparent clocks between the grandmaster clock and the slave clocks.